1. Field of the Invention
This invention relates to a magnetic recording disk cartridge, and more particularly to a magnetic recording disk cartridge containing a very thin magnetic recording disk of small diameter which is adapted to rotate at a high speed during recording and playback of video signals.
2. Description of the Prior Art
As is well known, a magnetic recording disk or floppy disk is composed of a flexible polyester sheet disk having both surfaces coated with a magnetic substance for recording digital information with the aid of a magnetic recording head, and the floppy disk has been widely used as a recording medium for computers because it is easy to handle and inexpensive to produce.
Recently, use of the floppy disk has extended to a still camera, in which the floppy disk is used in place of silver-halide coated film, taking advantage of the reusability of the magnetic recording medium in addition to those advantages as mentioned above. The floppy disk appropriate for use in a still camera must be small in size, that is, small in diameter and at the same time thin in thickness. Then, a casing for accommodating such a small-sized floppy disk can be reduced in size accordingly, and it has been proposed that a small-sized magnetic recording disk cartridge be made of a hard material and designed to allow the magnetic recording disk to rotate at a high speed. Such a small hard cartridge (hereinafter referred to as "cartridge") has made it possible to develop a 35 mm still camera using a magnetic recording medium in place of a roll of film.
Different from the conventional silver halide coated photographic film for exposure to light in taking pictures, a magnetic recording medium can be removed from the camera body before the whole recording area of the magnetic recording medium has been used, and then the unfinished recording medium can be put in the camera body again to use the remaining space for additional pictures. In this connection, a plurality of magnetic recording disks can be classified by subject, and then these magnetic recording disks can be selectively used for additional recording in the remaining blank space, thus permitting compilation of pictures by subject.
In this case, when the user removes a disk from the camera body, it is necessary for him to be able to know the number of pictures already taken and the number that can be additionally taken.